For an existing Long Term Evolution (LTE) system, only one carrier can be present in a cell and provided with the maximum bandwidth of 20 MHz, as illustrated in FIG. 1A.
For a Long Term Evolution-Advanced (LTE-A) system, there are significantly improved peak rates of the LTE-A system as compared with the LTE system. In particular, up to 1 Gbps in the downlink and 500 Mbps in the uplink are required for the LTE-A system. Apparently this demand has failed to be accommodated by the 20 MHz bandwidth. In order to enable the LTE-A system to comply with the requirement, the technology of Carrier Aggregation (CA) has been introduced, that is, a plurality of contiguous or non-contiguous carriers are aggregated together in the same cell to serve concurrently a user equipment if necessary to thereby provide a desired rate. Thus the LTE-A system is a multi-carrier system. In order to ensure the user equipment of the LTE-A system to be capable of operating on each aggregated carrier, there is no more than 20 MHz per carrier. The CA technology of the LTE-A is as illustrated in FIG. 1B.
In the LTE-A system in FIG. 1B, 4 carriers are aggregated. A base station can transmit data to the user equipment concurrently on the 4 carriers to improve the throughput of the system.
For the LTE TDD system, the User Equipment (UE) may feed back, in an uplink subframe, Acknowledgment/Non-Acknowledgement (ACK/NACK) information corresponding to a plurality of downlink subframes, that is, the UE feeds signaling (i.e., ACK/NACK) in an uplink subframe n back to the base station regarding whether data in a downlink subframe n−k needs to be retransmitted after demodulating and decoding the data in the downlink subframe, where kεK, and the values of the set K depend upon an uplink-downlink configuration of the system and a particular subframe number, particularly as depicted in Table 1.
TABLE 1Uplink feedback specified for downlink transmissionUplink-downlinkSubframe numberconfiguration01234567890——6—4——6—41——7, 64———7,64—2——8, 7, 6, 4————8, 7, 6, 4——3——11, 7, 66, 55, 4—————4——12, 11, 8, 77, 6, 5, 4——————5——13, 12, 11, 9,———————8, 7, 6, 5, 46——775——77—
A plurality of radio frames are arranged in sequence, that is, if the last subframe in a radio frame a is k, then the first subframe in a radio frame a+1 is k+1, and Table 1 depicts K corresponding to each uplink subframe taking only a radio frame as an example, where n−k<0 indicates a downlink subframe in a previous radio frame.
In a system of the LTE Rel-11 or later, in order to avoid interference with another Time Division Duplex (TDD) system, different TDD uplink-downlink configurations may be used for LTE cells in different bands, as illustrated in FIG. 1C, where a carrier 1 and a carrier 2 operate in a band A, and a carrier 3 operates in a band B, and a cell 1, a cell 2 and a cell 3 are cells respectively over the carrier 1, the carrier 2 and the carrier 3. There are identical TDD uplink-downlink configurations of the cell 1 and the cell 2, both of which are the uplink-downlink configuration 1, and a TDD uplink-downlink configuration of the cell 3, which is the uplink-downlink configuration 2, is different from those of the cell 1 and the cell 2. If these three cells are desired to be carrier-aggregated for the UE, then more than one TDD uplink-downlink configuration will be present across all the aggregated cells of the UE.
There has been absent so far of a solution to transmission of feedback information following aggregation of carriers with different TDD uplink-downlink configurations.